This project has three broad goals: (1) to improve early identification of autism, (2) to assess the efficacy of early intensive behavioral intervention (EIBI) for treatment of autism, and (3) to better understand individual child factors that account for variability in response to EIBI. The specific aims are: 1. To identify cognitive, social, neuropsychological, and electrical brain activity differences between 18-24 month old toddlers with autism versus 18-24 month olds with developmental (cognitive) delay (DD) and typical development. 2. To conduct a randomized study of EIBI to evaluate the efficacy of EIB! for improving outcomes of young children with autism, based on measures of cognitive, language, and social behavior. 3. To also evaluate the efficacy of EIBI for improving outcomes based on measures of brain activity. Our previous studies have shown that children with autism show atypical patterns of event related brain potentials in response to social stimuli. Given the emphasis on improving social behavior (e.g., eye contact) in EIBI, it is possible that EIBI may result in improved outcome on brain measures related to social processing. We will determine whether EIBI results in changes in brain activity such that, after treatment, children who have received EIBI will show more normal patterns of brain activity than those who do not receive such intervention. 4. To identify individual child factors that account for variation in response to EIBI in young children with autism. We hypothesize that three child factors will be important predictors of response to intervention. These are (1) IQ, (2) severity of autism symptoms, and (3) degree of early brain impairment, specifically, degree of medial temporal lobe dysfunction. A better understanding of factors related to response to early intervention would inform decisions regarding appropriate, individualized intervention methods and elucidate brain mechanisms involved in autism.